Because no man can be friends with a woman that he finds attractive. He always wants to have sex with her.
6 seconds sound clip from the When Harry Met Sally... movie soundboard.
You can hear this line at 00:12:31 in the Blu-ray version of the movie.
Quote context
[...]
- You're saying I'm having sex with these men without my knowledge?
- No, what I'm saying is they all wanna have sex with you.
- Do, too.
- Do, too.
- So you're saying that a man can be friends with a woman he finds unattractive.
- No, you pretty much wanna nail them, too.
- What if they don't wanna have sex with you?
- Well. I guess we're not gonna be friends then.
[...]